Schistosomiasis is a parasitic infection of man which is wide-spread in tropical countries. Schistosomiasis, known as biharziasis in Europe and as big belly disease in China, ranks as one of the most debilitating of tropical infections. Schistosomiasis is the result of infection with sexually mature and reproducing adult worms of any one of several closely related species of parasitic flat worms. The disease occurs where the parasite's specific small intermediate hosts are found. Approximately 250 million people are estimated to be infected, with about 5% of those displaying severe disease symptoms.
Human schistosomiasis is transmitted by water contact. The cercariae, or mobile larvae, emerge from the snail host and penetrate the skin of humans, where they rapidly develop into schistosomula. The schistosomula enter the blood stream and migrate to the lungs at 4 to 14 days. The schistosomula re-enter the blood stream migrating to the mesenteric or pelvis venules where they mate and lay eggs.
Often, upon infection, a rash occurs at the penetration site. Other symptoms include cough, fever, eosinophilia, and asthma-like symptoms. The adult worm grows to approximately 1.5 cm and can lay up to 500 eggs a day.
While there are some drugs which effectively counter infection, drug resistance as well as incomplete curing of worms creates recurrence problems. Effective vaccines are needed to provide lasting protection from these infections in areas of the world where water sources will continue to remain contaminated with fecal matter.